The problem of providing an apparatus for spray drying liquid and paste-like materials, which offer a high capacity at a low power consumption and hence also a high rate of spray drying, ensuring at the same time a high quality of the finished product, still remains to be adequately solved.
The high quality of the finished product implies a state of the atomized product, close to a monodispersed one, with a uniform size of the particles.
Attempts have been undertaken to solve the problem, if only in part, through the use of high-temperature products of combustion of a fuel-air mixture as the spraying medium in the apparatus for spray drying liquid and paste-like materials. This offers simultaneous atomization and drying of said materials, thereby intensifying the process of making the finished product.
There is known, for example, an apparatus for drying the above-specified materials by spraying in a steady-state flow, disclosed in the USSR Inventor's Certificate No. 157,279, Int. Cl..sup.2 BO58B 3/18, which apparatus comprises a chamber with a nozzle to introduce a high-velocity flow of the gaseous drying medium, a fuel-air mixture, and a means to inject a liquid or paste-like starting material, disposed on the axis of the nozzle.
In this apparatus, however, the atomized material has a non-uniform size of particles, which is due to the regularities of the process of decomposition of the starting material in a steady-state gas flow.
This causes difficulties in the subsequent heat treatment of the material and impairs the quality of the finished product. Moreover, utilizing the maximum temperatures of the combustion products in this apparatus is impossible because of a low heat resistance of the structural materials employed to manufacture the apparatus of such a type. This factor necessitates an additional mixing of the combustion products with air to lower the temperature, which entails a higher power consumption and impairs the technical and economic efficiency of the production process as a whole.
Closer in the technical substance and efficiency to the present invention is an apparatus for spray drying of liquid and paste-like materials in an unsteady flow of a gaseous drying medium, disclosed in U.S. Pat. No. 3,618,655, U.S. Cl. 159-4. The apparatus comprises a cylindrical chamber for combustion of a fuel-air mixture coming into the chamber in a pulsating flow, which chamber has a coaxial inlet pipe for the inflow of said mixture and a sprayer for injection of liquid or paste-like materials, said materials being injected along the centerline of said chamber in the direction of travel of the fuel-air mixture flow. At the opposite (with respect to the fuel-air mixture inlet pipe) end of said chamber, an exhaust pipe is mounted wherein simultaneous atomization and drying of the liquid or paste-like material are accomplished.
The apparatus functions as follows.
A fuel-air mixture is fed by portions into said chamber in a conventional manner and ignited, e.g., by a spark plug. The combustion of a portion of said mixture causes the pressure in said chamber to increase; the increased pressure acts upon, e.g., a check valve to shut off temporarily the access of the next portion of the fuel-air mixture. The combustion products rush into the exhaust pipe. The energy of the gas mass flowing out through the exhaust pipe creates a partial vacuum in the chamber so that the valve opens, the next portion of the fuel-air mixture fills the chamber and ignites from the residual combustion products. The filling, ignition, and exhaust processes follow one another at a definite frequency and amplitude. After the apparatus has come to a stable running, a liquid or paste-like starting material is injected into the chamber through the sprayer. The fuel-air mixture combustion products rushing at a high velocity into the exhaust pipe entrain the starting material and atomize it. The atomized material is dried due to contact with the combustion products.
This prior art apparatus provides for utilizing the maximum temperatures of the fuel-air mixture combustion products, but, as pointed out in said patent, features a very high level of noise, on the order of 130 dB, which prevents its commercial employment without special acoustical protection means and hence entails an extra cost. Moreover, some part of the liquid or paste-like starting material gets onto the exhaust pipe walls, which results in formation of a film or of coarse particles respectively and consequently reduces the rate of the drying and atomization processes as well as impairing the quality of the finished products.